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A fuel cell includes an electrolyte electrode assembly and a pair of
separators sandwiching the electrolyte electrode assembly. Each of the
separators is a single plate, and includes a plurality of trapezoidal
portions. First protrusions and second protrusions are provided on both
surfaces of the trapezoidal portion. The first protrusions contact the
anode of the electrolyte electrode assembly, and the second protrusions
contact the cathode of the electrolyte electrode assembly. A fuel gas
inlet for supplying a fuel gas into a fuel gas flow field is formed on
the trapezoidal portion. The fuel gas inlet is provided at an upstream
position away from an intermediate position of the electrolyte electrode
assembly in a flow direction of an oxygen-containing gas.

1. A fuel cell comprising a plurality of electrolyte electrode assemblies and a pair of separators sandwiching said plurality of electrolyte electrode assemblies, each of
said plurality of electrolyte electrode assemblies including an anode, a cathode, and an electrolyte interposed between said anode and said cathode, said separators each comprising a single plate, wherein first protrusions are provided on one surface of
said separator, and said first protrusions form a fuel gas flow field for supplying a fuel gas from a central region to an outer circumferential region of said plurality of electrolyte electrode assemblies along an electrode surface of said anode;
second protrusions are provided on the other surface of said separator, and said second protrusions form an oxygen-containing gas flow field for supplying an oxygen-containing gas in one direction from an outer circumferential region of said plurality of
electrolyte electrode assemblies along an electrode surface of said cathode; a channel member forming a plurality of fuel gas supply channels connected to a fuel gas supply unit is provided on said one surface of said separator or said other surface of
said separator; an exhaust gas channel for discharging a reactant gas consumed in reaction in said plurality of electrolyte electrode assemblies as an exhaust gas into the stacking direction of said plurality of electrolyte electrode assemblies and said
separators; said fuel gas supply unit is provided hermetically in said exhaust gas channel for supplying said fuel gas before consumption in the stacking direction; said fuel gas supply channels are connected between said fuel gas flow field and said
fuel gas supply unit, and extends along a surface of said separator which intersects said exhaust gas channel extending in the stacking direction; said fuel gas supply channels are connected to a fuel gas inlet for allowing said fuel gas to flow into
said fuel gas flow field through said fuel gas inlet; and said fuel gas inlet is provided at an upstream position away from an intermediate position of said plurality of electrolyte electrode assemblies in a flow direction of said oxygen-containing gas.

2. A fuel cell according to claim 1, wherein said exhaust gas channel is provided in a central region of said separator.

3. A fuel cell according to claim 1, wherein said fuel gas supply unit is provided hermetically in a central position of said exhaust gas channel.

4. A fuel cell according to claim 1, further comprising an oxygen-containing gas supply unit for supplying said oxygen-containing gas before consumption from said outer circumferential region of said plurality of electrolyte electrode
assemblies toward said oxygen-containing gas flow field.

5. A fuel cell according to claim 1, wherein one of said first and second protrusions is an annular protrusion and the other of said first and second protrusions is a mountain shaped protrusion.

6. A fuel cell according to claim 5, wherein said mountain shaped protrusion is disposed in a recess formed by said annular protrusion.

7. A fuel cell according to claim 6, wherein said mountain shaped protrusion and said annular protrusion are coaxial with each other.

8. A fuel cell according to claim 5, wherein the vertical cross section of said mountain shaped protrusion has a trapezoidal shape.

9. A fuel cell according to claim 6, wherein said recess has a perfectly circular shape.

10. A fuel cell according to claim 6, wherein said recess has an oval shape.

11. A fuel cell according to claim 1, wherein each of said plurality of electrolyte electrode assemblies has a substantially trapezoidal shape formed by dividing a substantially ring shaped body into areas of a certain angle; and said
plurality of electrolyte electrode assemblies are fanged along a circle concentric with a center of said separator.

12. A fuel cell stack formed by stacking a plurality of fuel cells in a stacking direction, said fuel cells each comprising a plurality of electrolyte electrode assemblies and a pair of separators sandwiching said plurality of electrolyte
electrode assemblies, each of said plurality of electrolyte electrode assemblies including an anode, a cathode, and an electrolyte interposed between said anode and said cathode, and having a substantially trapezoidal shape formed by dividing a
substantially ring shaped body into areas of a certain angle, said separators each comprising a single plate, wherein said fuel cell stack further comprises a tightening load applying mechanism for applying a tightening load to said fuel cells in the
stacking direction; a fuel gas supply unit extends through a central region of said fuel cells in the stacking direction, and said plurality of electrolyte electrode assemblies are provided in an outer region of each of said fuel cells; and a load
applied to said central region of said fuel cells in the stacking direction by said tightening load applying mechanism is larger than a load applied to said outer region of said fuel cells in the stacking direction by said tightening load applying
mechanism.

13. A fuel cell stack according to claim 12, wherein first protrusions are provided on one surface of said separator, and said first protrusions form a fuel gas flow field for supplying a fuel gas from a central region to an outer
circumferential region of said plurality of electrolyte electrode assemblies along an electrode surface of said anode; and second protrusions are provided on the other surface of said separator, and said second protrusions form an oxygen-containing gas
flow field for supplying an oxygen-containing gas in one direction from an outer circumferential region of said plurality of electrolyte electrode assemblies along an electrode surface of said cathode.

14. A fuel cell stack according to claim 13, wherein one of said first and second protrusions is an annular protrusion and the other of said first and second protrusions is a mountain shaped protrusion.

15. A fuel cell stack according to claim 14, wherein said mountain shaped protrusion is disposed in a recess formed by said annular protrusion.

16. A fuel cell stack according to claim 15, wherein said mountain shaped protrusion and said annular protrusion are coaxial with each other.

17. A fuel cell stack according to claim 14, wherein the vertical cross section of said mountain shaped protrusion has a trapezoidal shape.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell including an electrolyte electrode assembly and a pair of separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte
interposed between the anode and the cathode. Each of the separators is a single plate. Further, the present invention relates to a fuel cell stack formed by staking a plurality of fuel cells.

2. Description of the Related Art

Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The
electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.

In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O.sup.2-) move toward the anode
through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO.sub.2. Electrons released in the
reaction flow through an external circuit to the cathode, creating a DC electric energy.

In the fuel cell, various proposals for reducing the thickness of the separator and the number of components of the separator have been made in order to reduce the dimension in the stacking direction of the fuel cell. For example, as shown in
FIG. 17, according to the disclosure of Japanese Laid-Open Patent Publication No. 2002-75408, a separator 1 of a fuel cell includes a thin sheet of separator body 2, and a large number of first micro-protrusions 3 formed integrally on one surface of the
separator body 2, and a large number of second micro-protrusions 4 formed integrally on the other surface of the separator body 2. The first micro-protrusions 3 form a fuel gas flow field 6 between the separator 1 and a fuel electrode 5, and the second
micro-protrusions 4 form an oxygen-containing gas flow field 8 between the separator 1 and an air electrode 7.

However, according to the disclosure of Japanese Laid-Open Patent Publication No. 2002-75408, the fuel gas flow field 6 and the oxygen-containing gas flow field 8 are simply formed on both surfaces of the separator 1, and the supply of fuel gas
and the supply of the oxygen-containing gas are performed in a limited manner. That is, the fuel gas is simply supplied from one end of the fuel electrode 5 to the other end of the fuel electrode 5, and the oxygen-containing gas is simply supplied from
one end of the air electrode 7 to the other end of the air electrode 7. Therefore, the fuel cell cannot be used in wide applications. Depending on the shapes of the fuel electrode 5 and the air electrode 7, it may not be possible to supply the fuel gas
and the oxygen-containing gas uniformly to the entire electrode surfaces, respectively.

Further, since it is necessary to maintain the sealing performance for preventing the leakage of the fuel gas and the sealing performance for preventing the leakage of the oxygen-containing gas suitably, the sealing structure is complicated.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a fuel cell having a simple and compact structure in which the thickness in a stacking direction of the fuel cell is reduced, while maintaining the desired sealing performance.

A main object of the present invention is to provide a fuel cell in which the efficient power generation is achieved.

Another object of the present invention is to provide a fuel cell stack having a compact structure in which large electrical energy can be obtained.

According to the present invention, a fuel cell comprises an electrolyte electrode assembly and a pair of separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes an anode, a cathode, and an
electrolyte interposed between the anode and the cathode. Each of the separators comprises a single plate. First protrusions are provided on one surface of the separator. The first protrusions form a fuel gas flow field for supplying a fuel gas along
an electrode surface of the anode. Second protrusions are provided on the other surface of the separator. The second protrusions form an oxygen-containing gas flow field for supplying an oxygen-containing gas along an electrode surface of the cathode.

A channel member forming a plurality of fuel gas supply channels connected to a fuel gas supply unit is provided on the one surface of the separator or the other surface of the separator. The fuel gas supply channels are connected to a fuel gas
inlet for allowing the fuel gas to flow into the fuel gas flow field through the fuel gas inlet. The fuel gas inlet is provided at an upstream position away from an intermediate position of the electrolyte electrode assembly in a flow direction of the
oxygen-containing gas.

According to the present invention, the separator comprises one single plate, and the channel member forming the fuel gas supply channels is provided on the separator. With the simple structure, the dimension in the stacking direction of the
separator (thickness) is significantly reduced. Further, with the use of the channel member, it is possible to place the electrolyte electrode assembly in the oxygen-containing gas atmosphere. It is not necessary to prevent the leakage of the
oxygen-containing gas. Therefore, the number of sealing members for the oxygen-containing gas is reduced. Reduction in the production cost is achieved.

Further, since the fuel gas is supplied into the fuel gas flow field from the upstream position away from the intermediate position of the electrolyte electrode assembly in the flow direction of the oxygen-containing gas, the flow direction of
the fuel gas and the flow direction of the oxygen-containing gas are substantially the same.

In particular, in a substantially trapezoidal electrolyte electrode assembly, the fuel gas concentration in the outer region is high in comparison with fuel gas concentration in the inner region, and the area of the outer region is larger than
the area of the inner region. Therefore, in the outer region of the electrolyte electrode assembly, the flow rate of the fuel gas is low, and the reaction occurs smoothly. The area of the inner region of the electrolyte electrode assembly is small. In
the inner region, the flow rate of the fuel gas is high. Therefore, the fuel gas having the low concentration after the reaction can be discharged from the inner region of the electrolyte electrode assembly rapidly. Thus, the effective area used for
power generation is increased effectively. With the simple structure, the fuel cell can produce large electrical energy advantageously.

Further, it is preferable that the fuel cell further comprises an exhaust gas channel for discharging a reactant gas consumed in reaction in the electrolyte electrode assemblies as an exhaust gas into the stacking direction of the electrolyte
electrode assemblies and the separators. It is preferable that the fuel gas supply unit for supplying the fuel gas before consumption in the stacking direction is provided hermetically in the exhaust gas channel, and extends in the stacking direction,
and that the fuel gas supply channels are connected between the fuel gas flow field and the fuel gas supply unit, and extend along a surface of the separator which intersects the exhaust gas channel extending in the stacking direction.

It is preferable that the exhaust gas channel is provided in a central region of the separator. Further, it is preferable that the fuel gas supply unit is provided hermetically in a central position of the exhaust gas channel.

Further, it is preferable that the fuel cell comprises an oxygen-containing gas supply unit for supplying the oxygen-containing gas before consumption from the outer circumferential region of the electrolyte electrode assembly toward the
oxygen-containing gas flow field.

Further, it is preferable that one of the first and second protrusions is an annular protrusion and the other of the first and second protrusions is a mountain shaped protrusion.

Further, it is preferable that the mountain shaped protrusion is disposed in a recess formed by the annular protrusion. With this structure, the tightening load is efficiently transmitted to the electrolyte electrode assembly. Further, since
the volume of the gas flow field is not decreased excessively, the pressure loss is not increased excessively, and the desired power generation reaction is induced.

Further, it is preferable that the mountain shaped protrusion and the annular protrusion are coaxial with each other.

Further, it is preferable that the electrolyte electrode assembly has a substantially trapezoidal shape formed by dividing a substantially ring shaped body into areas of a certain angle. It is also preferable that a plurality of the electrolyte
electrode assemblies are arranged along a circle concentric with the center of the separator.

Further, according to the present invention, a fuel cell stack is formed by stacking a plurality of fuel cells in a stacking direction. Each of the fuel cells comprises an electrolyte electrode assembly and a pair of separators sandwiching the
electrolyte electrode assembly. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. The electrolyte electrode assembly has a substantially trapezoidal shape formed by
dividing a substantially ring shaped body into areas of a certain angle. Each of the separators comprises a single plate.

The fuel cell stack further comprises a tightening load applying mechanism for applying a tightening load to the fuel cells in the stacking direction. A fuel gas supply unit extends through a central region of the fuel cells in the stacking
direction, and the electrolyte electrode assembly is provided in an outer region of each of the fuel cells. A load applied to the central region of the fuel cells in the stacking direction by the tightening load applying mechanism is larger than a load
applied to the outer region of the fuel cells in the stacking direction by the tightening load applying mechanism.

In the fuel cell stack, the tightening load applying mechanism applies a large tightening load to the fuel gas supply unit to maintain the desired sealing performance for the fuel gas supply unit. Further, the tightening load applying mechanism
applies a relatively small tightening load to the electrolyte electrode assemblies such that the separators tightly contact the electrolyte electrode assemblies for improving the current collection performance without any damages in the electrolyte
electrode assemblies.

Each of the electrolyte electrode assemblies has a substantially trapezoidal shape. The electrolyte electrode assemblies are arranged in a substantially ring shape as a whole. Therefore, the length of the contact portion between the adjacent
electrolyte electrode assemblies is large, and the amounts of the fuel gas and the oxygen-containing gas which are not consumed, and flowing between the electrolyte electrode assemblies are reduced desirably. Thus, the effective area used for power
generation is increased effectively. With the compact structure, large electrical energy can be produced advantageously.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present
invention are shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a fuel cell stack formed by stacking a plurality of fuel cells according to a first embodiment of the present invention;

FIG. 2 is a cross sectional view showing part of a fuel cell system in which the fuel cell stack is disposed in a casing;

FIG. 15 is a plan view showing first and second protrusions that are different from the first and second protrusions shown in FIG. 7;

FIG. 16 is a perspective view showing first and second protrusions that are different from the first and second protrusions shown in FIGS. 7 and 15; and

FIG. 17 is a cross sectional view showing a separator of a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-75408.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view schematically showing a fuel cell stack 12 formed by stacking a plurality of fuel cells 10 according to a first embodiment of the present invention indicated by an arrow A. FIG. 2 is a cross sectional view showing
part of a fuel cell system 16 in which the fuel cell stack 12 is disposed in a casing 14.

The fuel cell 10 is a solid oxide fuel cell (SOFC) used in various applications, including stationary and mobile applications. As shown in FIGS. 3 and 4, the fuel cell 10 includes electrolyte electrode assemblies 26. Each of the electrolyte
electrode assemblies 26 includes a cathode 22, an anode 24, and an electrolyte (electrolyte plate) 20 interposed between the cathode 22 and the anode 24. For example, the electrolyte 20 is made of ion-conductive solid oxide such as stabilized zirconia.
Each of the electrolyte electrode assemblies 26 has a substantially trapezoidal shape which is formed by dividing a substantially ring body at certain angles. The electrolyte electrode assembly 26 includes a barrier layer at least on its outer
circumferential end for preventing entry of an oxygen-containing gas.

A plurality of, e.g., eight electrolyte electrode assemblies 26 are interposed between a pair of separators 28 to form the fuel cell 10. The electrolyte electrode assemblies 26 are concentric with a fuel gas supply passage (fuel gas supply unit)
30 extending through the center of the separators 28.

As shown in FIG. 3, for example, each of the separators 28 comprises a metal plate of, e.g., stainless alloy or a carbon plate. The separator 28 has a first small diameter end portion 32. The fuel gas supply passage 30 is formed at the center
of the first small diameter end portion 32. The first small diameter end portion 32 is integral with trapezoidal portions 36 through a plurality of first bridges 34. The first bridges 34 extend radially outwardly from the first small diameter end
portion 32 at equal angles (intervals). The shape of the trapezoidal portion 36 and the shape of the electrolyte electrode assembly 26 are substantially the same. The trapezoidal portion 36 includes side sections 37a, an arc shaped outer section 37b,
and a substantially straight inner section 37c. The width between the side sections 37a on both sides of the trapezoidal portion 36 is narrowed in the flow direction of oxygen-containing gas indicated by the arrow B.

As shown in FIGS. 3 and 5, each of the trapezoidal portions 36 has first protrusions 48 on its surface 36a which contacts the anode 24. The first protrusions 48 form a fuel gas flow field 46 for supplying a fuel gas along an electrode surface of
the anode 24. Each of the trapezoidal portions 36 has second protrusions 52 on its surface 36b which contacts the cathode 22. The second protrusions 52 form the oxygen-containing gas flow field 50 for supplying the oxygen-containing gas along an
electrode surface of the cathode 22 (see FIG. 6). As shown in FIG. 7, the first protrusions 48 and the second protrusions 52 protrude in opposite directions.

In the embodiment of the present invention, the first protrusions 48 are ring shaped protrusions, and the second protrusions 52 are mountain shaped protrusions (the second protrusions 52 have a conical shape having a flat top surface). The
second protrusions (mountain shaped protrusions) 52 are surrounded by the first protrusions (ring shaped protrusions) 48.

The second protrusions 52 are formed on the surface where recesses 53 corresponding to the first protrusions 48 are formed. Therefore, the second protrusions 52 are provided in the recesses 53.

In the embodiment of the present invention, the central axis L1 of the first protrusion 48 having the perfectly circular ring shape matches the central axis L2 of the second protrusion 52. Stated otherwise, the center of the first protrusion 48
is coaxial with the center of the second protrusion 52.

As shown in FIGS. 8 and 9, a plurality of the first and second protrusions 48, 52 are provided, and the height H1 of the first protrusion 48 is smaller than the height H2 of the second protrusion 52 (H1<H2). Therefore, the volume of the
oxygen-containing gas flow field 50 is larger than the volume of the fuel gas flow field 46. The first and second protrusions 48, 52 of these shapes may be formed by, e.g., press forming, an etching process or a cutting process.

The first protrusion 48 may be the mountain shaped protrusion, and the second protrusion 52 may be the ring shaped protrusion. In this case, it is preferable that the height of the ring shaped protrusion is larger than the height of the mountain
shaped protrusion.

As shown in FIGS. 3 through 6, the trapezoidal portion 36 has at least one, e.g., three fuel gas inlets 54 for supplying a fuel gas to the fuel gas flow field 46 from the central region of the anode 24. The positions of the fuel gas inlets 54
are determined by the shape of the trapezoidal portion 36, and the pressure of the fuel gas and the pressure of the oxygen-containing gas. The fuel gas inlets 54 may be provided at upstream positions away from the intermediate position of the
trapezoidal portion 36 in the flow direction of the oxygen-containing gas indicated by the arrow B. As shown in FIG. 5, for example, an intermediate position P in the radial direction of the trapezoidal portion 36 indicated by the arrow B, between the
outer section 37b and the inner section 37c is determined (.alpha..sub.1=.alpha..sub.2), and the fuel gas inlets 54 are positioned at intermediate positions between the intermediate position P and the outer section 37b.

A channel member 56 is fixed to the separator 28 by brazing or laser welding on a surface facing the cathode 22. As shown in FIG. 10, the channel member 56 includes a second small diameter end portion 58. The fuel gas supply passage 30 is
formed at the center of the second small diameter end portion 58. Eight substantially T-shaped second bridges 60 extend radially from the second small diameter end portion 58. Each of the second bridges 60 is fixed to the first bridge 34 of the
separator 28. Tip ends of the second bridges 60 are fixed to the surface 36b of the trapezoidal portion 36.

A plurality of slits 62 are formed on the second small diameter end portion 58 of the channel member 56. The slits 62 are formed radially on a surface of the second small diameter end portion 58 which is joined to the separator 28. The slits 62
are connected to the fuel gas supply passage 30. Further, the slits 62 are connected to a recess 64 formed in the outer circumferential region of the second small diameter end portion 58. The recess 64 prevents the flow of the brazing material, and
achieves the uniform flow of the fuel gas. A plurality of fuel gas supply channels 66 are formed between the first and second bridges 34, 60. Each of the fuel gas supply channels 66 is connected to the fuel gas flow field 46 through the slits 62 and
the recess 64. As shown in FIG. 11, the channel member 56 joined to each of the separators 28 has a curved cross section so that the second bridge 60 can be deformed elastically in the stacking direction indicated by the arrow A.

As shown in FIGS. 8 and 9, the oxygen-containing gas flow field 50 is connected to the oxygen-containing gas supply unit 68. The oxygen-containing gas is supplied in the direction indicated by the arrow B through the space between the outer
circumferential edge of the electrolyte electrode assembly 26 and the outer section (circumferential edge) 37b of the trapezoidal portion 36.

As shown in FIG. 8, insulating seals 69 for sealing the fuel gas supply passage 30 is provided between the separators 28. For example, the insulating seals 69 are made of mica material, or ceramic material. Exhaust gas channels 67 extend
through the fuel cells 10 in the stacking direction at positions internal from the respective trapezoidal portions 36.

As shown in FIGS. 1 and 2, the fuel cell stack 12 includes a plurality of fuel cells 10 stacked together, and circular end plates 70a, 70b provided at opposite ends in the stacking direction. The fuel cells 10 of the fuel cell stack 12 are
tightened together by a tightening load applying mechanism 72 in the stacking direction.

The tightening load applying mechanism 72 includes a first tightening unit 74a for applying a first tightening load T1 to a position near the fuel gas supply passage 30, and a second tightening unit 74b for applying a second tightening load T2 to
the electrolyte electrode assemblies 26. The second tightening load T2 is smaller than the first tightening load T1 (T1>T2).

The end plate 70a is insulated from the casing 14. A fuel gas supply port 76 is provided at the center of the end plate 70a. The fuel gas supply port 76 is connected to the fuel gas supply passage 30 in each of the fuel cells 10. The end plate
70a has two bolt insertion holes 78a. The fuel gas supply port 76 is positioned between the two bolt insertion holes 78a. The bolt insertion holes 78a are provided at positions corresponding to the exhaust gas channels 67 of the fuel cell stack 12.

Eight substantially circular openings 80 are provided along a circular line which is concentric with the fuel gas supply port 76. That is, the circular openings 80 are arranged at positions corresponding to the respective electrolyte electrode
assemblies 26. The circular openings 80 are connected to rectangular openings 82 extending toward the fuel gas supply port 76. The rectangular openings 82 are partially overlapped with the exhaust gas channels 67. Therefore, the exhaust gas is
discharged from the rectangular openings 82.

The end plate 70b is a conductive member. As shown in FIG. 2, a connection terminal 84 protrudes axially from the center of the end plate 70b, and the end plate 70b has two bolt insertion holes 78b. The connection terminal 84 is positioned
between the two bolt insertion holes 78b. The bolt insertion holes 78a are in alignment with the bolt insertion holes 78b. Two tightening bolts (tightening members) 86 are inserted into the bolt insertion holes 78a, 78b. The tightening bolts 86 are
insulated from the end plate 70b. Tip ends of the tightening bolts 86 are screwed into nuts 88 to form the first tightening unit 74a. The first tightening unit 74a applies the desired tightening load for tightening the fuel cells 10 between the end
plates 70a, 70b.

The connection terminal 84 is electrically connected to an output terminal 92a fixed to the casing 14.

The second tightening unit 74b is provided in each of the circular openings 80 of the end plate 70a. The second tightening unit 74b includes a presser member 94 as a terminal plate. The presser member 94 electrically contacts the end of the
fuel cell stack 12 in the stacking direction. One end of a spring 96 contacts the presser member 94, and the other end of the spring 96 is supported by an inner wall of the casing 14. The spring 96 has a spring load which is smaller than the first
tightening load T1. For example, the spring 96 is made of ceramics to prevent the influence of heat at the time of power generation, and to provide insulation.

A tubular conductor 98 is connected to an end of each presser member 94. The tubular conductor 98 and one end of a tightening bolt 86 are electrically connected through a conductive wire 100. The other end (head) of the tightening bolt 86 is
positioned near the connection terminal 84, and electrically connected to an output terminal 92b through a conductive wire 102. The output terminals 92a, 92b are arranged in parallel, and are adjacent to each other. The output terminals 92a, 92b are
also electrically insulated from each other and fixed to the casing 14.

The casing 14 has an air supply port 104 adjacent to the output terminals 92a, 92b. An exhaust port 106 is provided on the side of the other end plate 70a. A fuel gas supply port 108 is provided adjacent to the exhaust port 106. Therefore,
heat is exchanged between the exhaust gas and the fuel gas. The fuel gas supply port 108 is connected to the fuel gas supply passage 30 through a reformer 110 as necessary. A heat exchanger 111 is provided around the reformer 110.

Next, operation of the fuel cell stack 12 will be described below.

As shown in FIG. 3, in assembling the fuel cell 10, firstly, the separator 28 is joined to the channel member 56 on its surface facing the cathode 22. Therefore, each fuel gas supply channel 66 connected to the fuel gas supply passage 30 is
formed between the separator 28 and the channel member 56. Each fuel gas supply channel 66 is connected to the fuel gas flow field 46 through the fuel gas inlet 54 (see FIG. 8). The ring shaped insulating seal 69 is provided on each of the separators
28 around the fuel gas supply passage 30.

In this manner, the separator 28 is fabricated. The eight electrolyte electrode assemblies 26 are interposed between the separators 28 to form the fuel cell 10. As shown in FIGS. 3 and 4, the electrolyte electrode assemblies 26 are interposed
between the surface 36a of one separator 28 and the surface 36b of the other separator 28. Three fuel gas inlets 54 are positioned in the central region of each of the anodes 24.

A plurality of the fuel cells 10 are stacked in the direction indicated by the arrow A, and the end plates 70a, 70b are provided at opposite ends in the stacking direction. As shown in FIGS. 1 and 2, the tightening bolts 86 are inserted into the
respective bolt insertion holes 78a, 78b of the end plates 70a, 70b. Tip ends of the tightening bolts 86 are screwed into the nuts 88. Thus, the fuel cell stack 12 is produced. The components of the fuel cell stack 12 are tightened together by the
tightening load applying mechanism 72 in the stacking direction, and the fuel cell stack 12 is attached in the casing 14 (see FIG. 2).

Then, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply port 108 of the casing 14, and an oxygen-containing gas (hereinafter also referred to as the air) is supplied to the air supply port 104 of the casing 14. The
fuel gas flows through the reformer 110, and is supplied to the fuel gas supply passage 30 of the fuel cell stack 12. The fuel gas flows in the stacking direction indicated by the arrow A, and flows through the slit 62 of the separator 28 in each of the
fuel cells 10, and flows into the fuel gas supply channels 66 (see FIG. 8).

The fuel gas flowing along one of the fuel gas supply channels 66 between the first and second bridges 34, 60 flows into the fuel gas flow field 46 through the fuel gas inlet 54 of the trapezoidal portion 36. The fuel gas inlet 54 is positioned
at the substantially central position of the anode 24 in each of the electrolyte electrode assemblies 26. Alternatively, the fuel gas inlet 54 may be provided at an upstream position deviated from the central position of the anode 24 in the flow
direction of the oxygen-containing gas indicated by the arrow B. Therefore, the fuel gas is supplied to the central region of the anode 24 from the fuel gas inlet 54. The fuel gas flows from the central region of the anode 24 to the outer
circumferential region of the anode 24 (see FIG. 9).

The oxygen-containing gas is supplied to the oxygen-containing gas supply unit 68 in the outer circumferential region in each of the fuel cells 10. The oxygen-containing gas flows into the space between the outer circumferential region of the
electrolyte electrode assembly 26 and the outer circumferential region of the trapezoidal portion 36 in the direction indicated by the arrow B, and flows toward the oxygen-containing gas flow field 50. As shown in FIGS. 8 and 9, in the oxygen-containing
gas flow field 50, the oxygen-containing gas flows from one end of the outer circumferential region (outer region of the separator 28) to the other end of the outer circumferential region (central region of the separator 28) of the cathode 22 of the
electrolyte electrode assembly 26.

Thus, in the electrolyte electrode assembly 26, the fuel gas flows from the central region to the outer circumferential region of the anode 24, and the oxygen-containing gas flows in one direction indicated by the arrow B on the electrode surface
of the cathode 22 (see FIG. 9). At this time, oxygen ions flow through the electrolyte 20 toward the anode 24 for generating electricity by electrochemical reactions.

The fuel cells 10 are connected in series in the stacking direction indicated by the arrow A. As shown in FIG. 2, one of the poles is connected from the connection terminal 84 of the electrically conductive end plate 70b to the output terminal
92a through a conductive wire 90. The other pole is connected from the tightening bolts 86 to the output terminal 92b through the conductive wires 102. Thus, the electrical energy can be collected from the output terminals 92a, 92b.

After the fuel gas and the oxygen-containing gas are consumed in the reactions, the fuel gas and the oxygen-containing gas flow toward the outer circumferential regions of the anode 24 and the cathode 22, respectively, in each of the electrolyte
electrode assemblies 26, and are mixed together. The mixed gas flows as an exhaust gas into the exhaust gas channels 67 extending through the separators 28, and flows in the stacking direction. Then, the exhaust gas is discharged to the outside of the
casing 14 from the exhaust port 106.

In the first embodiment, the separator 28 is a single plate, and the first and second protrusions 48, 52 are provided on both surfaces 36a, 36b of each of the trapezoidal portions 36 of the separator 28. The first protrusions 48 contact the
anode 24, and the second protrusions 52 contact the cathode 22. The channel member 56 is fixed to the separator 28. The channel member 56 forms the fuel gas supply channels 66 connecting the fuel gas supply passage 30 and the fuel gas flow fields 46
through the fuel gas inlets 54.

Therefore, the oxygen-containing gas flows from the outer region to the central region of the fuel cell 10 in one direction indicated by the arrow B, and the fuel gas flows from the fuel gas inlets 54 of the anode 24 toward the exhaust gas
channels 67. Thus, not only is the dimension in the stacking direction of the separator 28 (thickness) significantly reduced, but also the fuel gas is uniformly distributed (supplied) to the entire electrode surface of the anode 24. With the compact
structure of the separator 28, the reactions for power generation occur uniformly over the entire electrode surfaces of the electrolyte electrode assembly 26. Therefore, the power generation can be performed stably.

Further, since the channel member 56 forms the fuel gas supply channels 66 hermetically, each of the electrolyte electrode assemblies 26 can be placed in the oxygen-containing gas atmosphere. No sealing material for sealing the oxygen-containing
gas supply unit 68 is required. The number of sealing members is reduced to simplify the sealing structure, and reduction in the production cost is achieved easily.

In the first embodiment, the fuel gas inlets 54 are provided at upstream positions away from the intermediate position of the electrolyte electrode assembly 26 in the flow direction of the oxygen-containing gas. The pressure of the
oxygen-containing gas in the exhaust gas channel 67 is low in comparison of the pressure of the oxygen-containing gas in the oxygen-containing gas supply unit 68. Due to the influence by the pressure of the oxygen-containing gas, in the
oxygen-containing gas flow field 50, the fuel gas tends to flow back to the center of the separator 28. Therefore, the flow direction of the fuel gas is substantially the same as the flow direction of the oxygen-containing gas.

In particular, in the substantially trapezoidal electrolyte electrode assembly 26, the fuel gas concentration in the outer region is high in comparison with fuel gas concentration in the inner region, and the area of the outer region is larger
than the area of the inner region. Therefore, in the outer region of the electrolyte electrode assembly 26, the flow rate of the fuel gas is low, and the reaction occurs smoothly. The area of the inner region of the electrolyte electrode assembly 26 is
small. In the inner region, the flow rate of the fuel gas is high. Therefore, the fuel gas having the low concentration after the reaction can be discharged from the inner region of the electrolyte electrode assembly 26 rapidly. Thus, in each of the
electrolyte electrode assemblies 26, the effective area used for power generation is increased effectively. With the compact structure, the fuel cell 10 can produce large electrical energy advantageously.

The electrolyte electrode assemblies 26 are arranged in a substantially ring shape as a whole. Therefore, the length of the contact portion between the side sections 37a of the adjacent electrolyte electrode assemblies 26 is large, and the
amounts of the fuel gas and the oxygen-containing gas which are not consumed, and flowing between the electrolyte electrode assemblies 26 are reduced desirably. Thus, the effective area used for power generation is increased effectively. With the
compact structure, large electrical energy can be produced advantageously.

In each of the electrolyte electrode assemblies 26, the flow direction of the fuel gas and the flow direction of the oxygen-containing gas are substantially the same as a whole. Therefore, concentric reaction distribution appears on the
separator 28. Further, concentric temperature distribution also appears on the separator 28 (the change in the temperature appears concentrically on the separator 28). Therefore, heat stress or heat distortion in the circumferential direction is
reduced effectively.

Further, in each of the trapezoidal portions 36, the height H1 of the first protrusions 48 is smaller than the height H2 of the second protrusions 52. It is because the flow rate (volume) and the pressure of the fuel gas flowing through the fuel
gas flow field 46 formed by the first protrusions 48 are smaller than the flow rate (volume) and the pressure of the oxygen-containing gas flowing through the oxygen-containing gas flow field 50 formed by the second protrusions 52. The height H1 of the
first protrusions 48 and the height H2 of the second protrusions 52 can be changed freely depending on the pressures of the fuel gas and the oxygen-containing gas supplied to the fuel cell 10, for example.

Further, in the first embodiment, the first and second bridges 34, 60 are provided in the exhaust gas channels 67. The fuel gas supply channels 66 formed between the first and second bridges 34, 60 extends along the surface of the separator 28
which intersects the exhaust gas channels 67 extending in the stacking direction. Therefore, the fuel gas flowing through the fuel gas supply channels 66 is heated by the waste heat effectively, and improvement in the thermal efficiency is achieved
advantageously.

Further, the exhaust gas channel 67 is provided in the central region of the separators 28. Therefore, the fuel gas is heated uniformly by the heat transmission from the central region. Thus, improvement in the power generation efficiency is
achieved. Further, the fuel gas supply passage 30 is provided hermetically in a central position of the exhaust gas channel 67. Therefore, the waste heat of the exhaust gas is used for heating the fuel gas supplied to the electrolyte electrode
assemblies 26. Therefore, improvement in the power generation efficiency is achieved.

In the first embodiment, as shown in FIG. 2, the fuel cells 10 of the fuel cell stack 12 are tightened together by a tightening load applying mechanism 72 in the stacking direction. The tightening load applying mechanism] 72 includes a first
tightening unit 74a for applying a first tightening load T1 to a position near the fuel gas supply passage 30, and a second tightening unit 74b for applying a second tightening load T2 to the electrolyte electrode assemblies 26. The second tightening
load T2 is smaller than the first tightening load T1.

Specifically, the large tightening load (first tightening load T1) is applied to the portion around the fuel gas supply passage 30 to maintain the desired sealing performance for the fuel gas supply passage 30. The relatively small tightening
load (second tightening load T2) is applied to the portions sandwiching the electrolyte electrode assemblies 26 such that the separators 28 tightly contact the electrolyte electrode assemblies 26 for improving the current collection performance without
any damages or the like in the electrolyte electrode assemblies 26.

FIG. 12 is an exploded perspective view showing a fuel cell 120 according to a second embodiment of the present invention. FIG. 13 is a cross sectional view showing a fuel cell stack 122 formed by stacking a plurality of the fuel cells 120.
FIG. 14 is a cross sectional view schematically showing operation of the fuel cell 120. The constituent elements that are identical to those of the fuel cell 10 according to the first embodiment are labeled with the same reference numeral, and
description thereof will be omitted.

A channel member 124 is fixed to each of the separators 28 of the fuel cell 120, on a surface facing the anode 24. The channel member 124 includes substantially T-shaped second bridges 126 fixed to the first bridges 34 of the separator 28. A
plurality of fuel gas supply channels 66 are formed between the first and second bridges 34, 126. Tip ends of the respective second bridges 126 terminate at positions near the centers of the anodes 24 of the electrolyte electrode assemblies 26. A
plurality of fuel gas inlets 128 are formed at the tip ends of the second bridges 126. The fuel gas flows through the fuel gas inlets 128 toward the anodes 24. The trapezoidal portions 36 of the separators 28 do not have any fuel gas inlet 54 of the
first embodiment.

In the second embodiment of the present invention, the fuel gas supplied to the fuel gas supply passage 30 flows along the fuel gas supply channels 66 between the separator 28 and the channel member 124. Then, the fuel gas flows toward the
anodes 24 through the fuel gas inlets 128 formed at the tip ends of the channel member 124.

Therefore, the fuel gas is supplied suitably and uniformly from the central region of the anode 24 to the outer circumferential region of the anode 24, and improvement in the power reaction efficiency is achieved advantageously. Further, it is
not necessary to provide the fuel gas inlets 54 in the trapezoidal portions 36 in each of the separators 28. Therefore, the separators 28 have the simple structure, and reduction in the production cost is achieved easily.

In the first and second embodiments, as shown in FIG. 7, the first protrusion 48 has a ring shape perfectly circular (annular shape). Alternatively, for example, as shown in FIG. 15, the first protrusion 48 (recess 53) may have an oval ring
shape. In this case, assuming that the intersection of the major diameter "a" and the minor diameter "b" is the center of the first protrusion 48, if an axis extending through the center matches the central axis of the second protrusion 52, "the first
and second protrusions 48, 52 are coaxial".

In an example shown in FIG. 7, the second protrusion 52 has a conical shape, and a flat surface on its top. The cross section of the second protrusion 52 in the horizontal direction (e.g., the top surface) has a perfectly circular shape.
However, the "mountain shaped protrusion" of the present invention is not limited in this respect. The mountain shaped protrusion may include a protrusion having a vertical cross section (cross section along the height of the protrusion) in a
trapezoidal shape. That is, the mountain shaped protrusion may include a protrusion having a horizontal cross section (e.g., the top surface) in an oval shape.

Further, in FIG. 7, the first and second protrusions 48, 52 are coaxial, i.e., the central axis L1 of the ring shaped first protrusion 48 matches the central axis L2 of the second protrusion 52. However, it is not essential that the central axis
L1 of the first protrusion 48 matches the central axis L2 of the second protrusion 52, i.e., the first and second protrusions 48, 52 are not necessarily coaxial with each other. For example, as shown in FIG. 16, the central axis L2 of the second
protrusion 52 may be deviated from the central axis L1 of the first protrusion 48. It is a matter of course that the first protrusion 48 may have an oval ring shape in this case, and the second protrusion 52 may have a horizontal cross section in an
oval shape.

While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit
and scope of the invention as defined by the appended claims.